Acoustic liquid processing involves the use of acoustic vibrational energy to treat a process liquid. Typical treatments include chemical reaction stimulation, sterilization, flotation enhancement, degassing, defoaming, homogenization, emulsification, dissolution, deaggregation of powder, biological cell disruption, extraction, crystallization, agglomeration and separation.
Typically, such treatments employ acoustic vibrational energy frequencies in the ultrasonic range (i.e. above the human hearing threshold of about 16 kHz). Accordingly, acoustic fluid processing is sometimes called "ultrasonic processing" or "power ultrasound". More recently, the term "sonochemistry" has been applied to liquid processing techniques which use acoustic vibrations of any frequency. This invention pertains to the use of acoustic vibrations of any frequency, but is particularly useful at frequencies above 10 kHz.
A large body of literature has been written on the use of acoustic liquid processing for various applications. See for example "Sonochemistry: Theory, Applications and Uses of Ultrasound in Chemistry", by T. J. Mason and J. P. Lorimer, Ellis Horwood Limited, 1988; and "Practical Sonochemistry", by T. J. Mason, Ellis Horwood Limited, 1991.
In many processing applications, it is desirable to create a phenomenon known as "cavitation" wherein the process liquid is subjected to intense acoustic energy. This creates small, rapidly collapsing voids in the liquid. Although the inventors do not wish to be bound by any specific theories, it is generally believed that extreme local temperatures (i.e. 5000.degree. K.) and pressures (i.e. 500 atm.) in the vicinity of the cavitational collapses are largely responsible for the processing action. Since the acoustic intensity necessary to produce cavitation becomes very large for acoustic frequencies above 100 kHz in most liquids, this type of processing is typically confined to frequencies below 100 kHz.
In some other processing applications, it is desirable to use forces associated directly with the acoustic field, such as acoustic radiation and agitation, to effect the processing action. Detailed aspects of these techniques and others are taught in literature such as U.S. Pat. Nos. 5,164,094 Stuckart; 4,673,512 Schram; 4,983,189 Peterson et al.; and 5,192,450 Heyman.
Prior art devices used in acoustic liquid processing have typically employed a flow-through duct arrangement to confine the process liquid within a selected treatment volume. A cylindrical flow-through duct arrangement is desirable in many applications, particularly those involving treatments requiring high acoustic intensities, because the acoustic energy is geometrically focused along the longitudinal axis of the cylinder. This has several advantages, including the ability to attain higher intensity acoustic vibrations within the focal region; confinement of intense cavitation away from equipment surfaces, thereby reducing surface erosion and transducer decoupling problems; and, facilitation of the use of catalysts, fixed solid reagents, or sources of electromagnetic radiation (i.e. ultraviolet light) within the focal region for maximum utilization of the cavitational energy.
Prior art acoustic liquid processing devices incorporating a cylindrical duct design for confining the process liquid within a selected volume include U.S. Pat. Nos. 2,578,505 Carlin; 3,056,589 Daniel; 3,021,120 Van der Burgt; 3,464,672 Massa; 4,369,100 Sawyer; 4,433,916 Hall; 4,352,570 Firth; and 3,946,829 Mori et al. European Patent specification 0 449 008 Desborough; and japanese patent 3-151084 Murata also disclose such devices.
Almost all prior art acoustic liquid processing devices have utilized piezoelectric or magnetostrictive transducers to generate acoustic vibrations for application to the process liquid. Although such transducer materials can very efficiently convert electrical energy to mechanical energy at fixed frequencies, they exhibit some disadvantages when used in acoustic liquid processing devices like those described above.
Consider for example liquid processing devices having cylindrical piezoelectric transducer configurations as disclosed in U.S. Pat. Nos. 3,464,672 Massa; or 3,056,589 Daniel; or, in Japanese Patent No. 3-151084 Murata. The rigid, fragile nature of most piezoelectric materials makes it difficult to manufacture large diameter transducers, which may restrict the volumetric capacity of the liquid processing device and/or the maximum acoustic intensity attainable along its longitudinal axis. Also, such transducers often require elaborate installation and mounting arrangements which add substantially to the cost of constructing and maintaining the liquid processing device. Also, large tensile stresses induced in the transducer may lead to mechanical fatigue.
Instead of attempting to fabricate a single cylindrical transducer structure, one may alternatively mount a plurality of small piezoelectric or magnetostrictive transducers at discrete locations spaced around the outside of the cylinder which contains the process liquid. U.S. Pat. Nos. 2,578,505 Carlin; 4,369,100 Sawyer; and, 4,433,916 Hall; and, European Patent Application No. 0 449 008 Desborough disclose such arrangements. This approach reduces problems with the transducer per se, compared to designs which use unitary cylindrical piezoelectric transducer elements. But, because it is difficult to couple a plurality of discrete transducers well with the resonant modes of the acoustic load (i.e. the process liquid), operating efficiency is typically reduced in such arrangements.
Furthermore, prior art liquid processing devices utilizing piezoelectric or magnetostrictive transducers require the transducer to operate at mechanical resonance, which precludes the generation of multiple or tunable frequencies by a single transducer, as may be desirable in some liquid processing applications.